If two straight lines incommensurable in square which make the sum of the squares on them medial, but the rectangle contained by them rational, be added together, the whole straight line is irrational; and let it be called the side of a rational plus a medial area. For let two straight lines AB, BC incommensurable in square, and fulfilling the given conditions [X. 34], be added together; I say that AC is irrational. For, since the sum of the squares on AB, BC is medial, while twice the rectangle AB, BC is rational, therefore the sum of the squares on AB, BC is incommensurable with twice the rectangle AB, BC; so that the square on AC is also incommensurable with twice the rectangle AB, BC. [X. 16] But twice the rectangle AB, BC is rational; therefore the square on AC is irrational. Therefore AC is irrational. [X. Def. 4].